ABSTRACT ? Project 1 Rhesus macaque (RM) experiments and human clinical trials collectively indicate that broad protection against HIV infection involves mechanisms of anti-envelope humoral immunity that reach beyond conventional neutralizing activity. The most likely possibilities are Fc receptor (FcR)-dependent effector functions, including antibody-dependent cell cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP) or antibody- dependent trogocytosis. Recent studies in murine models where Fc regions and Fc receptors (FcR) are species-matched indicate that Fc-FcR interactions play an important role in antibody-mediated protection against HIV even when Fab-mediated direct neutralizing activity is present. Nevertheless, whether RM and humans exhibit similar relationships between protection and FcR-dependent immunity remains unclear. For example, the RV144 clinical trial linked non-neutralizing anti-gp120 humoral immunity with decreased risk of infection while passive immunization of RM with non-neutralizing human anti-envelope antibodies has failed to block infection but instead decreased the number of transmitted viruses. Further, we found that RM antibodies raised against an RV144-like vaccine regimen primarily mediate monocyte-directed trogocytosis against HIV envelope; whereas immune sera from human vaccinees enables mainly NK-mediated killing. Such variances must be reconciled before RM can be accurately used to model how HIV vaccine candidates and anti-envelope antibodies will function in humans. Accordingly, the hypotheses for this Program are that: a) mechanisms of protective humoral immunity diverge in RM versus humans largely because of differences in FcR-dependent effector functions; b) the elucidation of these differences will facilitate optimization of the RM model for HIV vaccine development. Towards this goal, this Project will test a novel hypothetical model to explain how anti- HIV envelope antibodies variably activate FcR-dependent effector functions in vivo. The key features of the model are that: a) the formation of an HIV envelope immune complex allosterically induces a distinct Fc structure (Immune Complex Fc, ICFc); b) the ICFc structure is marked by altered FcR binding compared to free immunoglobulin; c) ICFc structures diverge between human and RM systems such that their respective FcR preferences differ. Thus, dominant effector mechanisms can differ between macaques and humans despite similar epitope targeting. Following our published work and preliminary data, this hypothesis will be explored by examining HIV epitopes that arise during early replication. These include broadly neutralizing epitopes as well as CD4-induced, transition state epitopes known to be potent targets for FcR-dependent anti- HIV activity against cell-bound virions. This hypothesis will be tested in two specific aims: Aim 1. Establish that HIV-1 envelope-IgG immune complexes have distinct, species-specific Fc?R binding profiles. Aim 2. Establish that antibody specificity and Fc?R binding profiles determine immune effector mechanisms in humans versus rhesus macaques.